The present invention relates to inkjet printing devices, and particularly although not exclusively to a method and apparatus for servicing a pen when mounted in a printing device.
Inkjet printing mechanisms may be used in a variety of different printing devices, such as plotters, facsimile machines or inkjet printers. Such printing devices print images using a colorant, referred to generally herein as xe2x80x9cink.xe2x80x9d These inkjet printing mechanisms use inkjet cartridges, often called xe2x80x9cpens,xe2x80x9d to shoot drops of ink onto a page or sheet of print media. Some inkjet print mechanisms carry an ink cartridge with an entire supply of ink back and forth across the sheet. Other inkjet print mechanisms, known as xe2x80x9coff-axisxe2x80x9d systems, propel only a small ink supply with the printhead carriage across the printzone, and store the main ink supply in a stationary reservoir, which is located xe2x80x9coff-axisxe2x80x9d from the path of printhead travel. Typically, a flexible conduit or tubing is used to convey the ink from the off-axis main reservoir to the printhead cartridge. In multi-color cartridges, several printheads and reservoirs are combined into a single unit, with each reservoir/printhead combination for a given color also being referred to herein as a xe2x80x9cpenxe2x80x9d.
Each pen has a printhead that includes very small nozzles through which the ink drops are fired. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporisation chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energised to heat ink within the vaporisation chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energised resistor.
To print an image, the printhead is scanned back and forth across a printzone above the sheet, with the pen shooting drops of ink as it moves. By selectively energising the resistors as the printhead moves across the sheet, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text). The nozzles are typically arranged in one or more linear arrays. If more than one, the two linear arrays are located side-by-side on the printhead, parallel to one another, and substantially perpendicular to the scanning direction. Thus, the length of the nozzle arrays defines a print swath or band. That is, if all the nozzles of one array were continually fired as the printhead made one complete traverse through the printzone, a band or swath of ink would appear on the sheet. The height of this band is known as the xe2x80x9cswath heightxe2x80x9d of the pen, the maximum pattern of ink which can be laid down in a single pass.
The orifice plate of the printhead, tends to pick up contaminants, such as paper dust, and the like, during the printing process. Such contaminants adhere to the orifice plate either because of the presence of ink on the printhead, or because of electrostatic charges. In addition, excess dried ink can accumulate around the printhead. The accumulation of either ink or other contaminants can impair the quality of the output by interfering with the proper application of ink to the printing medium. In addition, if colour pens are used, each printhead may have different nozzles which each expel different colours. If ink accumulates on the orifice plate, mixing of different coloured inks (cross-contamination) can result during use. If colours are mixed on the orifice plate, the quality of the resulting printed product can be affected. For these reasons, it is desirable to clear the printhead orifice plate of such contaminants and ink on a routine basis to prevent the build up thereof. Furthermore, the nozzles of an ink-jet printer can clog, particularly if the pens are left uncapped in an office environment.
In an off-axis pen, life goal is on the order of 40 times greater than a conventional non off-axis system, e.g. the printhead cartridges available in DesignJet(copyright) 750C color printers, produced by Hewlett-Packard Company, of Palo Alto, Calif., the present assignee. Living longer and firing more drops of ink means that there are greater probability that the printer print quality degrade and/or deviate along life. This requires finding better ways to keep functional and stable our printheads during long periods and large volumes of ink fired.
In order to maintain the quality of the printed output of the printer device it is important to improve the certainty that each instruction to the printhead to produce an ink drop from a nozzle of the plurality of nozzles does will produce such an ink drop (i.e. good servicing of the printhead and replacing nozzles out with working nozzles in performing error hiding).
In the present application, the term plot means a printed output of any kind or size produced by a printing device. For instance a plot could be a printed CAD image or a printed graphic image like a photo or a poster or any other kind of printed image reproduction.
In U.S. Pat. No. 5,455,606 it is described how a printer may adjusts servicing of the pen based on the result of the current drop detection step only. Before starting a plot these printers perform a drop detection on all the pens to detect if there are any non-firing nozzles (xe2x80x9cnozzles outxe2x80x9d). If a single nozzle out is detected in a pen, the printer triggers a so called automatic recovery servicing process for servicing the malfunctioning pen to recover the malfunctioning nozzle(s).
This process includes a sequence of 3 nozzle servicing or clearing procedures of increasing severity which are performed in sequence so long as some of the nozzles of the printhead fail to fire ink drops pursuant to ink firing pulses provided to the printhead or until all of the procedures have been performed.
At the end of each of these procedures a new drop detection is performed on the pen, to verify if the pen is fully recovered. If, according to the current result of the drop detection, it is not, the subsequent servicing procedure is performed. If, at the end of the 3 functions, the pen is still not fully recovered (i.e. at least one nozzles is still out) the user is reported to replace the pen or to disable the nozzle check. One big drawback of this system when implemented, e.g. as in DesignJet(copyright) 750 C printers, is that if the printer is not able to fully recover the failing nozzles or there are some unstable nozzles, the system will remain in this recovery servicing mode until the decease of the printhead, being forced, by the permanent nozzle out, to run this process at the beginning of each plot. This usually leads to either an unacceptable loss of throughput and printer productivity (because the printer stops and waits for an answer, the automatic recovery process is very time consuming, and causes a big waste of ink particularly when running the priming functions) or to excessive printhead replace or continue messages that users disable nozzle check via front panel, causing throughput losses.
European Patent Application no. 99 103283.0 in the name Hewlett-Packard Company (Docket number 60980059) describes a technique for servicing a printhead, by checking the status of the printhead by means of a drop detector sensing ink droplets fired by the nozzles of such a printhead. This technique monitors the more recent status of the nozzles and employs an incremental counter, reporting in a condensed way a number of historical statuses of the nozzles, to decide whether or not executing a recovery service on the printhead. In particular the recovery algorithm comprises 3 different servicing procedures (spitting, wiping, priming) which are applied in sequence, from the softer servicing (spitting) to the stronger one (priming), to the printhead. The decision to pass from one servicing procedure to the next one in the sequence is based on the monitored efficacy of the currently applied servicing procedure, i.e. if a servicing procedure is increasingly recovering nozzles, this is usually repeated; if not, a stronger servicing procedure is started to attempt the recovery of the still malfunctioning nozzles. However, monitoring only the efficacy of a servicing procedure, implies the fact that some non-efficacious procedures (sometime these may affect the lifetime of the printhead itself) are often performed and than abandoned. The performance of useless, or even damaging, servicing procedures is then increasing the length of the entire recovery algorithm. In addition such unneeded recoveries may generate wear in the nozzle plate and in the component of service station and possibly a waste of ink. Finally the execution of wrong servicing may generate additional defects in the printhead.
The specific embodiments and methods according to the present invention aim to improve the efficiency and the efficacy of the recovery process thereby improving printing quality and the functional lifetime of the plurality of nozzles.
According to an aspect of the present invention, there is provided a method of recovering a printhead, having a plurality of nozzles, mounted in an inkjet printing device for printing plots, said printing device is capable of performing a variety of servicing functions, said method comprises the following steps: (a) defining a set of causes of failures for said printhead; (b) checking if one or more nozzles of the printhead are failing; (c) identifying the cause of failure of a failing nozzle within said set, also by how the failure evolved over time; and (d) based on the identified cause of failure, performing an appropriate servicing function for recovering the failing nozzle.
The identification of what is causing the failure of the printhead allows to improving the efficiency and efficacy of the recovery process. Firstly, an appropriate recovery can be often identified before executing any additional recovery functions, so speeding up the entire process. Secondarily, by allowing to skip the unnecessary functions and to apply only the ones that are more likely to solve or improve the failure, this can reduce most of the problems generated by the execution these unneeded or wrong functions.
Preferably, the step of identifying comprises the step of monitoring how the failure evolved over time. Advantageously, the step of checking further comprises the step of storing in a memory support data representing the health status of the nozzle at the time the nozzle was checked, and said step of identifying the cause of the failure of a nozzle is based on examining a plurality of said data individually stored over time in said memory support.
Contrary to what suggested in the EP Application no. 99 103283.0 cited above, the collection of data relative to the failures is now stored individually and not incrementally, in order to gives to a pattern recognition algorithm enough details over the previous statuses of the nozzles. This allows to track the evolution of the failure and so an easier identification of the possible causes of the defect(s) of the nozzle(s) or the printhead.
Typically, said data comprises a health code representing if the nozzle was working or failing at the time the nozzle was checked.
Preferably said step of identifying the cause of failure comprises, based on the evolution of the health of the nozzle over time, the step of generating a plurality of failure codes, representatives of the cause of failure of the nozzle.
This provides a very convenient way to assign to a nozzle the cause of its failure, which is important for identifying the appropriate servicing function to apply to the nozzle.
In a preferred embodiment the step of identifying the cause of a failing nozzles comprises the step of examining data stored over time in said memory relative to said failing nozzles and to other nozzles located in the vicinity of said failing nozzle.
In this way it is improved the recognition of failures, and so the efficacy of the associated recovery, which (a) affect more than a single nozzle; (b) are not stable over a nozzle or a group of nozzles, but that move along the printhead.
Advantageously, the set of causes of failures includes one or more of the following causes: internal contamination, external contamination, Bubbles, Start-up, Starvation, Bad pen, Punctual nozzle out, Valley, continuing aberrant, each causes being characterised by a unique evolution of the of the failure.
The correspondence between how a failure can evolve over time and a cause of the failure gives a more effective way of performing a pattern recognition of the different causes.
In a further preferred embodiment the appropriate servicing function for a first nozzle with an internal contamination failure is replacing, while generating a print mask for printing a plot, said first nozzle and at least one neighbour nozzle of said first nozzle with one or more working nozzles.
More preferably the appropriate servicing for a second nozzle with a continuing aberrant failure is replacing, while generating a print mask for printing a plot, said first nozzle with one or more working nozzles.
Viewing a second aspect of the present invention, there is also provided a plurality of recovery functions for recovering an inkjet printing device comprising a printhead, having a plurality of nozzles, and a servicing unit capable of applying said plurality of recovery functions to said plurality of nozzles characterised by the fact that each recovery function of said plurality of recovery functions is associated to at least one cause of failure of nozzle, said at least one cause of failure is identified also by how the failure evolved over time.
Viewing a third aspect of the present invention, there is also provided a computer program comprising computer program code means performing the following steps when said program is run on an inkjet printing device comprising a printhead, having a plurality of nozzles, and a servicing unit capable of applying said plurality of recovery functions to said plurality of nozzles: (a) enabling the device to check if one or more nozzles of the printhead are failing; (b) identifying the cause of the failure of a failing nozzle within a defined set of causes of failures for said printhead, also by how the failure evolved over time; and (c) based on the identified cause of failure, enabling the servicing unit to perform an appropriate servicing function for recovering the nozzle which is failing.
Viewing a forth aspect of the present invention, there is also provided an inkjet printing device for printing plots comprising a printhead, having a plurality of nozzles, a servicing unit capable of applying recovery functions to said plurality of nozzles characterised by comprising a plurality of recovery functions for recovering said device, where each recovery function of said plurality of recovery functions is associated to at least one cause of failure of a nozzle, the at least one cause of failure also is identified by how the failure evolves over time.